Support for custom profiles for serf agent

Currently serf provides local, lan and wan profiles. Parameters in these profiles are somewhat conservative(for example ProbeTimeout can be 99 percentile of RTT. But it is set to 200ms in local although RTT is much less~ 1-2ms in local network).
This is an enhancement request to support providing custom profile parameters via a json file to serf so that failure detection can be controlled by operators(i.e fast detection vs conservative detection)

example:
--customProfileFile="custom.json"

cat custom.json
{
"ProbeTimeout":"200 * time.Millisecond",
"SuspicionMult" :2,
"ProbeInterval" : "200 * time.Millisecond"
}

Thanks.

Start by locating the existing local, lan, and wan profile definitions and the agent option handling for --customProfileFile. Compare those parameters with custom.json, then verify that supported custom values control failure-detection timing without changing the built-in profiles.

Written by the indexing model from the issue text.